This invention relates to a method of controlling FR-based four-wheel drive vehicles, more specifically a method of controlling four-wheel drive vehicles provided with a rotation transmission device on a power line for driving the rear wheels of the vehicle for carrying out changeover between transmission and shutoff of the driving force to the front wheels.
FIG. 11 shows an example of an FR-based four-wheel drive vehicle. With this four-wheel drive vehicle, power transmitted from an engine 1 to a transmission 2 is transmitted to a rear wheel propeller shaft 3 to drive the rear wheels 4. A rotation transmission device 10 is provided on the rear wheel drive line. The rotation transmission device 10 changes over between two-wheel drive and four-wheel drive. When the four-wheel drive is selected, the rotation of the rear wheel propeller shaft 3 is transmitted to a front wheel propeller shaft 5 to drive the front wheels 6.
The rotation transmission device 10 comprises, as shown in FIG. 12, a two-way clutch 11 and an electromagnetic coil 12 for controlling engagement of the two-way clutch 11. In the two-way clutch 11, cam surfaces 14 are formed on the outer periphery of an inner ring 13 coupled to the rear wheel propeller shaft 3. Engaging elements 17 comprising rollers are mounted between the cam surfaces 14 and a cylindrical inner surface 16 of an outer ring 15. An elastic force of a switch spring 19 is imparted to a retainer 18 for retaining the engaging elements 17 to retain the retainer 18 at an engagement-released position where the engaging elements 17 are not in engagement with the cam surfaces 14 and the cylindrical inner surface 16.
The electromagnetic coil 12 is arranged so as to axially oppose the retainer 18. Between the electromagnetic coil 12 and the retainer 18, a friction flange 20, prevented from rotating relative to the outer ring 15, and an armature 21 are mounted. The armature 21 is prevented from turning relative to the retainer 18 but is axially movable. When the electromagnetic coil 12 is energized, the armature 21 is attracted to the friction flange 20, so that due to the relative rotation between the retainer 18 and the inner ring 13, the engaging elements 17 will engage the cam surfaces 14 and the cylindrical inner surface 16, thereby transmitting the rotation of the inner ring 13 to the outer ring 15.
After deenergizing the electromagnetic coil 12, when the rotating speed of the outer ring 15 decreases below that of the inner ring 13, the engaging elements 17 will return to the engagement-released position by the elasticity of the switch spring 19.
Between the outer ring 15 of this rotation transmission device 10 and the front wheel propeller shaft 5 shown in FIG. 11, a transmission mechanism 7 such as a chain transmission is provided to transmit the rotation of the outer ring 15 to the front wheel propeller shaft 5.
Thus, by engaging the two-way clutch 11 by energizing the electromagnetic coil 12 of the rotation transmission device 10, the four-wheel drive vehicle shown in FIG. 11 can be changed over from two-wheel drive to four-wheel drive.
The four-wheel drive vehicle provided with this rotation transmission device 10 has a mode changeover lever operated by a vehicle driver. For control for automatically changing over between two-wheel drive and four-wheel drive according to the traveling state of the four-wheel drive vehicle with the AUTO mode selected by the mode changeover lever, as shown in FIG. 11, an arrangement is known in which the numbers of revolutions of the rear wheels 4 and the front wheels 6 are detected by ABS sensors S11 and S12 and the difference in their numbers of revolutions or change in the numbers of revolutions is used (JP patent publications 8-175206 and 11-201195).
In driving the four-wheel drive vehicle shown in FIG. 11, though it is a rare case, if reverse drive is selected in which a range in a direction opposite to the traveling direction is selected, for example, in an automatic transmission (AT) vehicle, by putting in the reverse range (R range) during travel in the drive range (D range), the front wheels 6 tend to rotate in the traveling direction due to inertia of the vehicle, while the rear wheels 4, which are the driving wheels, tend to rotate in a direction opposite to the direction in which the vehicle is traveling, namely, in the direction opposite to the front wheels 6.
In such a case, on a high-μ road, i.e. a road that is high in the friction coefficient (μ) between the tires and the road surface, such as asphalt road, the engine may stop. But on a low-μ road, such as on a snow-covered or icy road, since the load on the rear wheels is much smaller than on a high-μ road, even if reverse drive is selected while traveling at several tens of kilometers per hour, the rear wheels 4 will rotate in a reverse direction without engine stop.
At this time, if the two-way clutch 11 of the rotation transmission device 10 is engaged, the front wheels 6, too, will rotate in the reverse direction together with the rear wheels 4, so that it is possible to keep traveling.
In a control in which two-wheel drive and four-wheel drive are automatically changed over from one to the other according to the travelling state of a four-wheel drive vehicle provided with the rotation transmission device 10, if the judgment of engagement as described above is carried out only with the difference between the numbers of revolutions of the front and rear wheels 4 and 6, or change in the numbers of revolutions detected by means of rotation sensors S13 and S14 for detecting the numbers of revolutions of the propeller shafts 3 and 5 of the front and rear wheels 4 and 6, or with ABS sensors S11 and S12 for detecting the numbers of revolutions of the front and rear wheels 4 and 6, a threshold has to be increased to distinguish from the number of revolutions or change in the rotation during normal travel. Thus, when the two-way clutch 11 is engaged by energizing the electromagnetic coil 12, the difference between the revolutions of the front and rear wheels 4 and 6 may be too large, so that engagement shock may occur when the engaging elements 17 engage the cam surfaces 14 and the cylindrical inner surface 16.
Such a problem may also occur with an FR-based four-wheel drive vehicle of a manual transmission (MT) vehicle provided with a rotation transmission device.
An object of this invention is to provide a method of controlling an FR-based four-wheel drive vehicle provided with a rotation transmission device having a two-way clutch and an electromagnetic coil for controlling engagement of the two-way clutch, which permits reverse travel without engagement shock on a low-μ road, which is free of error in judgment of reverse travel on a high-μ road, and which will not cause a tight corner braking.